1. Field of the Invention
The present invention relates to a reflective or transflective liquid crystal display device which displays images by using outside light, and more particularly relates to a liquid crystal display device having minute irregularities provided on a surface of a reflective film.
2. Description of the Prior Art
A liquid crystal display device has advantages that the device is thin and light and has low power consumption since the device can be driven at a low voltage. Thus, the liquid crystal display device is widely utilized in various electronic devices. Particularly, an active matrix liquid crystal display device in which a TFT (thin film transistor) is provided as a switching element for each picture element is as excellent in display quality as a CRT (cathode-ray tube). Thus, the active matrix liquid crystal display device is widely used for a display of a TV, a personal computer or the like.
A general liquid crystal display device has a structure in which liquid crystal is sealed between two substrates disposed so as to face each other. In one of the substrates, TFTs, picture element electrodes and the like are formed. In the other substrate, color filters, common electrodes and the like are formed. Hereinafter, the substrate in which the TFTs, the picture element electrodes and the like are formed is called a TFT substrate, and a substrate disposed so as to face the TFT substrate is called a counter substrate.
Liquid crystal display devices include the following three types, including: a transmission liquid crystal display device which displays images by using a backlight as a light source and using light transmitted through a liquid crystal panel; a reflective liquid crystal display device which displays images by utilizing reflection of outside light (natural light or lamplight); and a transflective liquid crystal display device which displays images by using a backlight in the dark and utilizing reflection of outside light in bright light.
The reflective liquid crystal display device requires no backlight and thus has an advantage that power consumption is lower than that of the transmission liquid crystal display device. Moreover, in a well-lighted location, the reflective liquid crystal display device or the transflective liquid crystal display device, both of which utilize outside light, can often display images better than the transmission liquid crystal display device which utilizes the backlight can.
Meanwhile, in the reflective liquid crystal display device and the transflective liquid crystal display device, if a film which reflects light (a reflective film) has a flat surface, a range within which clear images can be seen (a viewing angle) is extremely narrowed. At the same time, there occurs a problem of reflection and the like. Therefore, it is required to scatter light by providing minute irregularities on the surface of the reflective film.
There has heretofore been proposed a method for forming minute irregularities on the surface of the reflective film. For example, Japanese Patent Laid-Open No. Hei 5 (1993)-173158 describes that irregularities are formed on a surface of an organic insulating film (polyimide film) by use of a photolithography method and a dry etching method and a reflective film is formed thereon. Moreover, U.S. Pat. No. 2,990,046 describes that irregularities are formed by utilizing at least one of a metal film, an insulating film and a semiconductor film, all of which are used for forming a switching element (TFT), and a reflective film is formed thereon with an insulating film interposed therebetween.
However, the inventors of the present application consider that the conventional technologies described above have the following problems. Specifically, the technology disclosed in Japanese Patent Laid-Open No. Hei 5 (1993)-173158 requires a step of applying a photosensitive resin (photoresist) onto the organic insulating film, an exposure and development step and a dry etching step. Thus, along with an increase in the number of steps, manufacturing costs are increased and the yield is lowered.
In the technology described in U.S. Pat. No. 2,990,046, the metal film, the insulating film and the semiconductor film are laminated and etched by use of the photolithography method. Thus, the irregularities are formed simultaneously with formation of the TFT. Thereafter, an insulating film is formed on the entire surface and the reflective film is formed thereon. By forming the irregularities simultaneously with formation of the TFT as described above, an increase in the number of manufacturing steps can be avoided. However, in this technology, density of the irregularities depends on resolution of photolithography. Thus, it is difficult to form the irregularities with high density.
Moreover, in a part of the embodiment described in U.S. Pat. No. 2,990,046, a glass substrate is etched. However, if the glass substrate is etched, impurities contained in the glass substrate are eluted to contaminate liquid crystal. Thus, display quality may be significantly impaired.